I think the suggestion for one size smaller is right on- perhaps one that is thin material, vs a heavy jersey cotton. If you're talking about an undershirt, I'd suggest basic Hanes because tee shirts get stained and pitted out so quickly.
However I also am very fond of Calvin Klein tees- they fit my body very well and I feel confident and happy about how I appear to others- which always makes me feel better and then my posture is better, and I stand open and chest protruding, which makes me look better